Master Boot Record format by error

There is nothing inherently wrong with using an hdd formatted with MBR on the Mac. macOS should have no problem accessing any files that you have stored on that drive, barring some sort of hardware issue.

> What problem do you encounter when you try to access your files?

> You say Disk Utility does not help. What did you try?

> What sort of files have you copied to the drive?

> How did you copy those files to that drive?

> Assuming you have "Show external disks" enabled, does the drive appear to mount properly on the desktop?

If a simple reboot of your Mac doesn’t fix things, see if the problem still happens in Safe mode (press and hold the shift key down at startup). It can take much longer to safe boot (5 min) so be patient.

How to use safe mode on your Mac - Apple Support 

Safe mode very often corrects weird software behavior by forcing the OS to do cache clearing and other housekeeping. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.

Does the problem persist while in Safe mode?

Often not required, but consider allowing the Mac to "soak" in Safe mode for an hour to allow maintenance routines and processes time to complete, then exit safe mode by restarting your Mac normally and re-evaluate the issue again.

If you formatted a drive that already had your data on it, sadly all of that data is lost. There is no recovering from that without great expen$e.

If you formatted a drive using MBR and then, after that, copied files to the drive, I believe you should still have access to those files on that drive.

What was your order of operations here?

What drive are you using? Sounds like the drive is bad, or you have an issue with the cable, adapter/dock/hub, or even port on the computer. If this drive is a USB stick or SD Card, then there is a good chance it is bad, especially a USB stick since the quality of USB sticks is extremely poor...even ones from respected name brands.

Make sure to disconnect all other external devices. Connect the drive directly to the computer.

Do you have another computer available which can read the file system you used on that drive?

FYI, it is generally best to use GUID partitions these days since MBR partitions do have a drive size limitation. GUID partitions are much easier to work with. About the only time MBR partitions are needed today is when the drive will be used by an extremely old computer & operating systems, or for USB sticks & SD Cards which will be connected to another device such as TV, car, camera, etc.

Error message problem