For space reasons, I have a Batocera installation on an SD Card. Unfortunately, my laptop BIOS doesn't support booting from an SD card. The only drives it will boot from are USB, SSD and HDD.
Is there any way to boot a new OS from the SD card using my already installed Ubuntu system?
If your laptop is from ~2012 or newer it has UEFI, not BIOS. So, all OSes should be installed in UEFI mode and only one ESP (EFI System Partition) is required irrespective of where the OSes are actually installed.
That means you can, theoretically, have the bootloader in the internal drive and have the system partition in the SD card. However many SD card readers require an initialization that's different from internal drives (PATA, SATA, NVMe, etc.) so the aforementioned setup may not work due to the drive not being available yet when needed. And, generally speaking, SD cards are too slow for such usage.
Unfortunately this whole preamble is just an academic rambling because Batocera Linux is a self-contained solution that works like any other Linux live session, so, not actually installed anyway, it's just an image file decompressed and written to a target drive. The computer must be able to boot from the external drive where Batocera runs from and if yours can't your only options are to use an USB stick instead, preferably USB 3.x as fast as you can get, or use your existing SD card in a known good USB adapter which is recognized by the firmware like any other USB stick.
