Can I use Dropbox as a Time Machine backup device?

dialabrain and Ronasara are correct.

Virtual hard drives or "clouds" cannot be used as TimeMachines, the way TimeMachine was engineered. It was meant for external, physical, hard drives.

If you're in the market for one, here are the hard drives that Apple sells on their site:

External Hard Drive - Apple

If you decide to go elsewhere to buy one, make sure it can be formatted to fit a Mac! This is extremely important.

Here are some thoughts to consider.

• Having a reliable backup is essential to avoid future data loss.
• Time Machine, or another backup, app is designed to securely do our backups reliably.
• Backup apps are designed to work backing up to an external disk.
• The external disk should ideally be two to three times as large as the data to be backed up.
• External disks are reasonably priced these days and a number of reliable brands are available.

The short answer is no.

I wouldn't be able to tell you definitively yes or no. But if you're using iCloud (hopefully iCloud+) and DropBox, I would tentatively say, yes.

However this won't solve your problem of having a TimeMachine backup to your Mac.

Here's a link to iCloud+, if you're unfamiliar:

iCloud+ plans and pricing - Apple Support

What is iCloud+? - Apple Support

NOTE: Having iCloud+ will not increase the internal storage space on your device. This is a commonly misunderstood topic.

If you have a device with 256GB of internal storage, subscribing to iCloud+ will not increase that storage...so you may still run out of space to store Photos, Apps, etc. on your device. But all that information will be safe in iCloud, if you find out you need a new device with more storage at some point.

You're very welcome!