SAM Coupé disk
The SAM Coupé disk (3 1/2", double sided, double density) was the floppy disk format used in the SAM Coupé computer, a late-1980s British home computer that imitated the Sinclair ZX Spectrum. The disk drives on that computer were also able (with appropriate software) to read PC-DOS 720K format disks.
Disks for the SAM Coupé had 2 sides, 80 tracks per side, and 10 sectors per track. Each sector stored 512 bytes, making a total capacity of 800K, of which 20K was taken up by directory information.
A +D filesystem was used, based on the one used by the Sinclair, but with some additional file types. The disk operating system was called SAM Coupé ProDOS (not related to Apple's ProDOS) and also labeled CP/M 2.2, so it was apparently based on CP/M.
Disk data could be damaged if the disk was left in the drive when powering up or down the system.
3 1/2" disks are actually 90 mm wide, but are almost universally referred to as "3 1/2 inch" disks even in countries that use the metric system.