LaCie d2 Professional mirroring

I have a 2016 iMac (which is running OS Catalina (10.15.2) and was running out of space on my hard drive, so I bought a LaCie D2 Professional from the Apple Store to act as the external hard drive for my Photo Library. This then freed up 250GB on my iMac's hard drive. All good. However, the LaCie is creating a mirror file back on my iMac, so all the space is used up again. I know I can stop it mirroring and I have, but if I were to delete the mirrored Photo Library on my iMac, will it still be OK on the D2? Any advice much appreciated, as despite contacting LaCie support twice via email, I've had no response.