Thanks for that. I hadn't come across the idea of a sync byte. For anyone else, the binary representation of E5 is 11100101. No matter where you start reading you're always going to know whether you are reading from the start of the byte or not. Contrast with null (00000000) its impossible to know where you've started from.

This has a bit of info ftp://ftp.apple.asimov.net/pub/apple_II/documentation/misc/disk_encoding.doc.txt