Yes, initialise mounts the card, but the "upload" the option remains greyed out. Printer is a wanaho duplicator i3 v1 (marlin).
Remote upload to SD is the missing link in my procedure for remote printing (e.g. power on, slice, upload, print, power down again - all over the network) - My connection is just not reliable enough to print multi-hour prints from printer memory. I get connection failures in about 1 in 20 prints. I'm therefore currently printing prints over an hour long from the SD card (manually, using the machine's interface).
I've checked and the "feature" in Octoprint to Enable SD is ticked.
I also saw from much older threads that some folk with marlin boards tweaked their octoprint config.yaml to add "assume SD card present" - but not sure whether that was an old fix?
Would it help if I grabbed a serial log (like mentioned here)?
Your printers firmware may not allow you to write to the SD remotely. Another thought is what size is the SD card and did you format it as FAT32, exFAT etc?
Yes, a serial.log always helps a ton when facing issues in printer communication, and OctoPrint not recognizing that the firmware supports SD cards certainly is such an issue.
Be aware that "Upload to SD" is, to be kind, a less than optimal solution. Each line of the gcode file is passed over the USB serial connection just like it would be if printed directly so its only slightly faster.