External drives · Drive formats
Why can't I use a FAT32 (MS-DOS) drive?
Short answer: FAT32 can't hold the shortcuts Storage Studio relies on and can't store files over 4 GB, so it's marked not suitable. Back up the drive, then erase it as APFS in Disk Utility to use it.
FAT32 is an old Windows format that can't hold the shortcuts Storage Studio leaves behind, and it can't store files larger than 4 GB — so moved apps and big folders would break. Choose Drive marks it not suitable and won't let you select it.
To use the drive, copy off anything you need, then open Disk Utility, select the drive and click Erase, choosing APFS. Erasing deletes everything on the drive, so check you've saved what matters first.