Yes, the Canon EOS R5 Mark II is compatible with the Lexar Professional 1667x 64GB SD.
Canon EOS R5 Mark II's SD slot (Slot 2) accepts Lexar Professional 1667x 64GB SD directly. Lexar Professional 1667x 64GB SD is V60-rated, clearing the SD slot's 4K Long-GOP floor (up to 119.88fps) but not the 8K Long-GOP floor (needs V90) — no setting or firmware update changes a card's rated speed class. Also, its own card-performance-requirements table shows every RAW, SRAW, and Intra-codec recording mode (XF-HEVC S / XF-AVC S High Quality Intra, Standard Intra, Light Intra) has no SD-card entry at all — those modes are CFexpress-only by design, not just speed-limited. Only Long-GOP (LGOP) compressed modes are SD-eligible.
Limitation
Cannot record 8K Long-GOP (needs V90) or any RAW/SRAW/Intra-codec mode (CFexpress Type B, VPG400 only) on the SD slot. Works for 4K Long-GOP up to 119.88fps and all lower-tier modes.
“Standard LGOP 119.88 CFexpress 2.0 V60 100.00”
Last verified:
Compatibility inferred from spec rule (citations above are manufacturer-verified)