Using NAS for Time Machine back ups

Thank you so much Barney for this reply. I bought the G-Drive as it was for sale in the Apple Store and the spec said 'Time Machine compatible'

That's when you connect it to the back of your Mac. You are connecting it to an unsupported router. The drive itself has nothing to do with the problems you are seeing.

Or to put it another way, what “Time Machine over SMB” NAS drive would you recommend?

Any drive will work with Time Machine, as long as it is connected to a supported "interface."

I don't know of any NAS that specifically states it supports that specification. They all will claim to allow you to backup to them using Time Machine, but they do that either through their hacked version of AFP or SMB--but they don't specify that their SMB implementation supports the specification. If you can determine which SMB file server they use, you can check the version to see if that version of the SMB server supports the spec. Most use Samba, so if they are running an unaltered version later than 4.8, it should support the spec. If their setup guide mentions any use of AFP, you should stay away from it. Apple never licensed AFP, so their AFP implementation is not true AFP, was never supported by Apple, and indicates they already play fast and loose with the facts.

I've always backed up over the network to a drive connected to a Mac Mini, previously a 2012 and now a 2020 M1. Never had issues with that setup, whether AFP or now SMB.

Hook up the drive to your iMac, partition it so you can backup both iMac to one partition and the MacBook to the other.

From the iMac Sharing System Preferences, share out the "network" partition and set it to be a Time Machine destination in File Sharing (ctrl-click on the drive and choose advanced options). If your user on the MacBook is the same as one on your iMac, you can use that to connect. Otherwise, create a Sharing Only user in Users & Groups and use those credentials. You'll have to give that user read/write on the shared partition.

The source and destination do not have to be the same file system format. It doesn't restore the "file system," it restores the files.

A network storage device can be any format known. The file server that talks to the client machines does the writing to disk, not Time Machine.

Network backups are written to a Sparse Bundle Disk Image "file." It will be formatted however Time Machine wants it to be formatted. On a directly connected drive, Time Machine uses APFS (case-sensitive) since Big Sur. It is an entirely different backup scheme than the old HFS+ concept. The sparse bundle disk image will be formatted APFS and could exist on just about any file system.

The problems with backing up to a NAS or your "NAS-like" setup is the file server isn't capable of doing the necessary error-checking required of a Time Machine backup. Apple published a specification for backing up over SMB that the NAS must support in order to have reliable backups. Those checks were built into AFP, but not into the Netatalk hack that NAS devices used to simulate AFP. Whatever the MX4200 uses to file share over SMB must support the "Time Machine over SMB" specification.

If it uses Samba, then it would need version 4.8 or later. If it doesn't use Samba, it would need to specify that it has incorporated the "Time Machine over SMB" specification in its SMB implementation.