You try to create Acronis Survival Kit on a USB disk larger than 2TB.
Survival Kit creation wizard shows the following warning:
Attention! Your computer is running in BIOS mode which limits bootable disks to be MBR with 2TB maximum size. If you proceed, the selected external disk will be converted from GPT to MBR, shrunk to 2TB, and you will not be able to use the remaining disk space beyond 2TB.
Your installed Windows system is running in BIOS mode, which restricts bootable disks to be MBR. MBR scheme restricts bootable partitions to maximum size of 2TB.
To keep all the disk space on USB drive, cancel Acronis Survival Kit creation and create a regular Bootable Media on a DVD or flash drive, instead of Survival Kit. You can still use the external drive for storing your backups.
Question: my system is UEFI-capable, and it is just the installed Windows runs in CSM (BIOS) mode. Why not keep the external disk GPT so that I could boot it in UEFI mode?
Answer: Even if the system supports UEFI and the external USB could be booted in UEFI mode, if you clone or restore the system, Acronis True Image would adjust its bootability for the mode in which the media is currently booted (UEFI), making the system unbootable in CSM (BIOS) mode.