How do I partition a Seagate 5 TB external drive for Time Machine and data storage on a new iMac 4?

You should not use an external drive for both data storage and Time Machine backups.

If the drive is formatted using APFS, you can have one APFS Volume for Time Machine, and one for storage of other files. Apple mentions that in a footnote as a possibility for advanced users.

Back up your Mac with Time Machine - Apple Support

That doesn't make it a good idea.

Keep your backups on separate drives from the master copies of your data. Make backups of all data that you care about. Better yet, make two backups – backup drives themselves can fail, and Murphy's Law says that the time you are likely to discover that a backup has failed is when you most need it.

Can I partition this drive without losing the files I have saved there?

No. Partitioning will wipe the drive.

I don't see a way to get into the Disk Manager on this new iMac 4,

On a Mac, it's Disk Utility. Open Launchpad, the icon in your dock looks like this:

and enter the app name in the search field at the top of the launchpad window.

Like Bob, I see having Time Machine share a drive is a potential problem. I separate them.

Guess my age is catching up with me.

Nah, you're good here, and among kindred spirits. Many of the senior contributors here are just that—senior. I have a 12-year old MediCare card!

+1

You need 2 external drives. 1 drive for data storage and 1 drive for Time Machine.

Part 1 of 3

Connect a storage device to your Mac

Connect an external storage device, such as a USB drive or Thunderbolt drive, to the appropriate port on your Mac. Identify the ports on your Mac.

• Use a storage device with at least twice the storage capacity of your Mac. For example, if your Mac has 1TB of storage, your backup disk should ideally have at least 2TB of storage. If Time Machine needs more storage space, your Mac will let you know.
• Use your Time Machine backup disk only for Time Machine backups, not for storage of other files.*Part 2 of 2

Part 2 of 3

The very nature of Time Machine Backup Utility is to Control the entire Physical drive and to make the drive a Read Only

This is so Only TM Backup can access and write to the drive

Part 3 of 3

To truly protect your non replaceable Data

Have a 3-2-1 Rescue Plan in place and always current

3 Backups using 2 methods and 1 off site incase of natural disaster or un-natural disaster.

Each of the above should be done to a Dedicated Single Purposed External Drive 

Below link is intended to augment what TM Backup does 

https://bombich.com

ringspa wrote:

8GB isn't that much frankly, you're probably better off keeping the 8GB on the iMac with the rest of your data and then back up everything to the Seagate, no additional drive needed. Keeping it simple is usually a good strategy. Ideally add an external backup service like BackBlaze and you're in good shape for any failure/disaster.

An aside to your Central Issue

To get the replies to your question in chronological order by Date 

Click on your User Name  >>   >> " Edit Profile and Preferences "

Scroll way down and look for >> >> " Other Preferences "

Change the >> >> " Default thread sort " to Oldest 

This is mentioned because your above reply, is actually a a reply to yourself @ ringspa

veehbJ wrote:

One more question: If I partition the external drive (APFS) and have multiple volumes. Would it be ok to run Time Machine in one volume and use the other volume for data archiving? I'm having trouble understanding why Time Machine needs its own drive.

The data loss problem is the same whether you have partitions or volumes it doesn’t matter they’re still all on the same desk. But only you can decide how valuable your data is and whether you wanna keep it isolated or bundle it all together to lose it all at once.

veehbJ wrote:

One more question: If I partition the external drive (APFS) and have multiple volumes. Would it be ok to run Time Machine in one volume and use the other volume for data archiving? I'm having trouble understanding why Time Machine needs its own drive.

Those archived files seem to be important to you. Those files should also be backed up, but backing them up using TM to the same physical drive is no backup at all. If the drive fails or is lost/stolen, then the archive and its backup are gone at the same time.

Here is an Apple article for TM where they do mention & advise against creating & using an APFS volume for data storage on a TM backup drive (it is a small footnote at the bottom of the article):

Back up your Mac with Time Machine - Apple Support

2 more cents here

Putting ALL your eggs in one basket and you drop the basket . . Scrambled eggs

veehbJ wrote:

I have a new iMac 4, 16 GB DRAM and 512 GB storage. I want to use Time Machine. It is replacing a 10 year old iMac running Mac OS 11 Big Sur.

I connected a Seagate 5 TB USB 3.0 External Drive (Seagate Rescue Drive) and have moved nearly 8 GBytes of old historical files I want to save from my old iMac, but they are not on the new iMac 4.

I want to keep these historical files on this external drive and also want to run Time Machine on this drive. Can I partition this drive without losing the files I have saved there? I think I need a different partition to run Time Machine. I'd like to create a new partition for Time Machine and leave the rest of the disk as it is.

You shouldn't use a partition. You should use an APFS Volume. All volumes in the same container (partition) share all of the storage space. That way, neither side restricts you if it needs more space (until the drive is full). But that assumes the drive is already formatted APFS or Mac OS Extended (can be converted to APFS).

Should I copy these archive files to the disk drive on my new iMac 4 in a temporary folder, and then partition the backup drive, and then move them back to the backup drive? Probably the safe thing to do.

I see a recommendation to use SMB rather than AFP for Time Machine.

Is this a network storage device (NAS)? You said it was an external drive which means it wouldn't use SMB or AFP. If it is a NAS, SMB is the preferred method, but if it is an old NAS, SMB might not work. Then again, AFP might not work as the NAS networking isn't actually AFP but a reverse engineered knockoff.

If I reformat this backup disk for SMB, can I still store files there.

SMB is a networking protocol, not a disk format.

As to why you shouldn't store files with a backup, if the drive fails, you don't have a backup for the other files, unless you make a separate backup for those, but then why not just get another drive for them.

8GB isn't that much frankly, you're probably better off keeping the 8GB on the iMac with the rest of your data and then back up everything to the Seagate, no additional drive needed. Keeping it simple is usually a good strategy. Ideally add an external backup service like BackBlaze and you're in good shape for any failure/disaster.