When an A4000D Rev B have an external drive connected the internal drive stops working. Tries to boot on startup, but gives up after 3 times trying to read the disk. When the external drive is disconnected the internal FDD works as expected including boot. Also if the internal drive disconnected (by cable) the external drive starts working and shows as DF0: When both drives are connected none works, but they react to disk change, however at disk insertion both disks show up as “DF0:?????”.
Solution: Check jumper J352 (next to CIA U300) and see if it’s jumpered correctly. (Postion 1-2). This jumper controls the SEL signal, if jumpered 1-2 external FDD will have drive ID DF2: if jumpered 2-3 external drive will act as DF0: hence if both drives are connected in this position both will ‘act’ as DF0: causing the problem.