SCPDU5 was originally distributed using a custom disk format and Simon Cooke's prototype of MaxiDOS+.

MaxiDOS+ is presumed to have used the non-standard 'extended' tracks 80 and 81 on the physical media to store the DOS - standard MGT format only normally used 80 tracks so this means the DOS lives in these 20 sectors giving at least an extra 8-10kB to store normal programs in the standard format area of the disk.

These two tracks at 10 sectors per track gives 510bytes*20 = 10,200bytes (or possibly 512bytes*20 = 10,240bytes depending of whether the last two bytes were used as the next track/sector indicators) to store the DOS file and uses a custom boot block in the place of the first sector of track 4 to load it from.  

The originally archived copy of this disk only had the first 80 tracks archived so will not boot on a real machine or an emulator although you can do a BOOT1 on any standard disk and then swap in SCPDU5 and hit F9 and it will work as expected.

In order to make this disk bootable a copy has been made with Samdos2 in standard format however to make space an advert for ZAT has had to be removed from the Adverts section.

The bootblock sector which was captured on the original image is included here since as it is overridden by the first sector of the DOS file,  as is the missing advert is attached here "ZAT.AD" and is a code file starting at 65536, 7,409 bytes long and with an autostart address of 65536.

Dan Dooré 2024