You may want to use disk imaging tools such as my favourite WinHex for capturing forensically sound disk images from arbitratily attached USB/SATA/mSATA/M.2/SAS/etc. harddrives even if you do not have a hardware based write-blocker device. In order to prevent the operating system from switching the just attached disks to Online mode and mounting any file systems, you should configure the following registry values:
NoAutoMount = REG_DWORD = 1
SanPolicy = REG_DWORD = 3
Note that the NoAutoMount value goes really directly into the mountmgr registry key, while the SanPolicy value must be set in the Parameters subkey of the partmgr driver.
If you have the registry configured this way, the newly attached disk drives remain in the default Offline mode which means that thay are read/only.
If you want to switch the disks to the writable Online mode, you can always do so with diskpart's command online disk. Note although that making disk Online means immediate mounting any respective filesystem even if no disk letter may be assigned yet.
If you wanted to mount any file system while keeping the disk in read/only mode, you can achieve this with diskpart's command attributes disk set readonly prior to switching the disk into the online mode.
Thus having the disk in the offline mode means always read/only, while having the disk in the online mode may mean read/only or writable, depending on the disk's attribute setting which you can change yet before making the disk online.
A sample DISKPART transaction may look like this:
REM :the previous command listed your disks, the newly attached disk should be offline, note its number
select disk XX
REM :select the number of your offline attached disk instead of typing XX
REM :the previous command should have displayed some attributes, mainly the fact that the disk is in read/only state
attributes disk set readonly
REM :we make the read/only setting permanent for the selected disk by storing this information in the local computer registry
REM :note that this does not modify anything on the disk yet and note also that the setting stays in the local computer registry
REM :and does not roam with the disk if unplugged and moved to another computer
REM :makes the disk online allowing file systems to be mounted, although the disk remains in read/only mode and thus the file systems
REM :are read only as well. No disk letters assigned yet due to the NoAutoMount registry value
REM :only now disk letters assigned, the disk and its file systems still remain in read/only mode