Anyone who cares has probably worked all this out by now, but as a newcomer it didn’t all seem to be available in the one place…

USB2 vs USB3 device - I’ve only had success with USB2. Anything USB3 just gave a device error.

Device size - I’ve had success with 16/32Gb flash drives. Now using a 120Gb SSD in a USB2 enclosure.

I’ve had best results if I make the device as “Legacy” and clean as possible:
From Windows, open DISKPART from a CMD window.
Find the correct device with “List Disk” and select with “Select Disk <disk>”. DO NOT SELECT THE WRONG DISK - you’ll accidentally wipe the wrong drive (including your Windows drive).
“Clean” erases everything on the selected disk - partitions, the lot. This is irretrievable, so check you’ve selected the right drive. Then check again, and again.
“Convert MBR” converts the drive from newer GPT to legacy MBR partition tables
“Create Partition Primary” creates a new partition

Then you can format temporarily as NTFS and assign a drive letter. This then enables you to reformat as FAT32 using a custom formatter (available online) as the drive is too big for Windows to do it.

The system likes mp3 format with minimal metadata tags.
The best program I’ve found for this is Foobar2000, with optional LAME MP3 encoder installed. This allows batch encoding from original format to mp3, up to 320kbps constant bit rate (highest quality). It also allows batch editing of metadata to remove any tags that may confuse the car. I remove everything except for artist, album, track name, track number.

The car certainly copes with structures at least two levels deep - I organise by a folder per artists, then subfolders of individual albums within each artist folder.

SSD in a USB2 enclosure is a revelation. Much faster to access - 50+Gb of albums found and ready to access before I’ve got my seatbelt on. And a lot faster to get onto the device in the first place! 2011 4.4 TDV8 Autobiography (winter)
2005 LS430 (summer)